1. Field of the Invention
The present invention relates to a time information-acquiring apparatus that receives a standard time radio wave and acquires the time information thereof and a radio wave timepiece comprising the time information-acquiring apparatus.
2. Description of the Related Art
In recent years, for example, in Japan, Germany, England, and Switzerland, transmitting stations transmit a standard time radio wave of a low frequency. For example, transmitting stations in Fukushima and Saga in Japan transmit amplitude-modulated standard time radio waves of 40 kHz and 60 kHz, respectively. The standard time radio wave includes a code string forming a time code indicating the date and time and is transmitted every 60 seconds. That is, the period of the time code is 60 seconds.
A clock (radio wave timepiece) that receives a standard time radio wave including a time code, extracts the time code from the received standard time radio wave, and corrects the time has been put to practical use. A receiver of the radio wave timepiece includes a band-pass filter (BPF) that receives the standard time radio wave through an antenna and extracts only a standard time radio wave signal, a demodulator that demodulates an amplitude-modulated standard time radio wave signal using, for example, envelope detection, and a processor that reads a time code included in the signal demodulated by the demodulator.
The processor according to the prior art is synchronized with the rising edge of the demodulated signal and performs binarization with a predetermined sampling period to acquire time code output (TCO) data with a unit time length (one second), which is a binary bit string. The processor measures the pulse width (that is, the time of a bit 1 or the time of a bit 0) of the TCO data, determines any one of a marker M code, a position marker code, a binary 0 code, and a binary 1 code based on the measured pulse width, and acquires time information based on the determined code string.
The processor according to the prior art performs processes, such as a second synchronization process, a minute synchronization process, a process of acquiring a code, and a process of determining matching, during the period from the start of the reception of the standard time radio wave to the acquisition of the time information. When each of the processes is not appropriately terminated, the processor needs to start the processes from the beginning. Therefore, in some cases, the processor sometimes needs to start the processes from the beginning several times due to the influence of noise included in the signal. For such instances, it takes a very long time to acquire time information.
The second synchronization detects the rising edge of a code at an interval of one second among the codes indicated by the TCO data. It is possible to detect a portion in which a position marker P0 arranged at the end of a frame and a marker M arranged at the head of the frame are continuously arranged by repeatedly performing the second synchronization. The portion in which the marks are continuously arranged appears at an interval of one minute (60 seconds). Within the TCO data, the marker M shows the position of the head frame data. The detection of the position of the marker is referred to as minute synchronization. The head of the frame is recognized by the minute synchronization. Therefore, after code acquisition starts to acquire one frame of data, a parity bit is checked to determine whether the data has an improper value (the date and time have improper values) (matching determination). For example, since the minute synchronization is for detecting the head of the frame, 60 seconds are required in some cases. Of course, multiples of 60 seconds are required to detect the heads of several frames.
In Jpn. Pat. Appln. KOKAI Publication No. 2005-249632 (corresponding to US 2005/0195690 A1), the demodulated signal is binarized at a predetermined sampling interval (50 ms) to obtain TCO data and a list of data groups comprising binary bit strings is obtained every one second (20 samples). The apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-249632 compares the bit string with the templates of binary bit strings indicating a marker code M, a position marker code, a code 1, and a code 0, calculates a correlation therebetween, and determines which of the marker code M, the position marker code P, a binary 1 code, and a binary 0 code corresponds to the bit string.
In the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-249632, the TCO data, which is a binary bit string, is acquired and matched with the template. When the field intensity is weak or a large amount of noise is mixed with the demodulated signal, many errors are included in the acquired TCO data. Therefore, it is necessary to provide a filter which removes noise from the demodulated signal or to finely adjust the threshold of an AD converter, in order to improve the quality of the TCO data.
The process of determining which of the marker code M, the position marker code P, the binary 1 code, and the binary 0 code corresponds to data with a unit time length (one second) is insufficient to detect the head of the second or the head of the minute, and it is necessary to perform the determining process again based on the determination result. When the head of the second or the head of the minute is not appropriately detected, it is necessary to perform the process again.